Question

The maximum number of isomers (including stereoisomers) that are possible monochlorination of the following compound, is

• A. 4
• B. 5
• C. 6
• D. 7

Answer 1

Answer: (D)

7

Answer 2

The given compound is 3-methylpentane. Monochlorination can occur at different types of hydrogen atoms, leading to different structural isomers. We need to identify these distinct positions and then determine the number of stereoisomers for each.

The structure of 3-methylpentane is: $CH_3$ (a) | $CH_3$ (b) - $CH_2$ (c) - $CH$ (d) - $CH_2$ (e) - $CH_3$ (f) | H (g)

There are four distinct types of hydrogen atoms:

1. Hydrogens of the methyl group attached to C3 (position a).
2. Hydrogens of the terminal methyl groups of the ethyl chains (positions b and f).
3. Hydrogens of the methylene groups of the ethyl chains (positions c and e).
4. The hydrogen atom attached to the central carbon (C3, position g).

Let's analyze the products of monochlorination:

1. Chlorination at methyl group (a): Product: 1-chloro-3-methylpentane. The carbon at position 3 (original d) becomes a chiral center. This yields 2 stereoisomers (R and S enantiomers).

2. Chlorination at terminal methyl groups (b or f): Product: 1-chloro-3-methylpentane. The carbon at position 3 (original d) becomes a chiral center. This also yields 2 stereoisomers (R and S enantiomers). The set of stereoisomers formed from positions (a) and (b)/(f) are identical, resulting in 2 unique isomers of 1-chloro-3-methylpentane.

3. Chlorination at methylene groups (c or e): Product: 2-chloro-3-methylpentane. The carbon at position 2 (original c) becomes a chiral center. The carbon at position 3 (original d) is also a chiral center. With two chiral centers, there are $2^2 = 4$ stereoisomers (R,R; R,S; S,R; S,S). These are distinct from the isomers formed in step 1 and 2.

4. Chlorination at the methine hydrogen (g): Product: 3-chloro-3-methylpentane. The carbon at position 3 is bonded to two identical ethyl groups, so it is not a chiral center. This yields only 1 structural isomer with no stereoisomers.

The total number of distinct isomers (including stereoisomers) is the sum of unique isomers from each type of chlorination: 2 (from 1-chloro-3-methylpentane) + 4 (from 2-chloro-3-methylpentane) + 1 (from 3-chloro-3-methylpentane) = 7.

Therefore, the maximum number of isomers is 7.